1. Field of the Invention
The present invention relates to interconnect structures in semiconductor devices. More specifically, the present invention relates to an interlevel dielectric stack containing a dense layer and a porous layer of the same chemical composition.
2. Description of the Related Art
Current copper back end of the line interconnect fabrication requires the use of a copper cap, graded adhesion layer and intra/interlevel dielectric and SiCOH stack. As the stack and the interlevel dielectric thickness become smaller, the effective dielectric constant, k, of the stack becomes higher reducing the advantage of using an ultra-low k dielectric. Furthermore, due to weak mechanical properties of the ultra-low k, flopping of the inter-level dielectric stack has been observed in high aspect ratio structures during fabrication. In addition, the electrical breakdown of the porous SiCOH is worsening as the thickness is reduced below 30 nm.
A new high modulus dielectric-cap structure with low effective k is required for enabling the fabrication of the interconnect structure of 14 nm and beyond technology with low effective k values. Performance of integrated circuits and their density has been improved continuously through the shrinking of the active devices according to Moore's law. Until the 0.25 μm technology node, this was accomplished using the same materials, that is, silicon semiconductor, silicon oxide dielectrics and aluminum, as the interconnect metal. At the 0.25 μm technology node this paradigm changed because the increased RC delay of the electrical signals through the interconnect became a barrier to further improvement of the VLSI performance and new materials had to be introduced to reduce the RC.
Copper was introduced at the 0.22 nm technology node to reduce both resistance and capacitance by appropriate line scaling relative to aluminum in a hierarchical design. Further reduction of the capacitance could be realized much later through the introduction of low-k plasma enhanced chemical vapor deposition (PECVD) SiCOH dielectrics and SiCNH copper cap with a dielectric constant k=2.7-3.0 and k=5.3 at the 90 nm-65 nm nodes, after many other low-k dielectrics <2.4 has fail integration effort at 32 nm and 22 nm node to lower mechanical modulus (<10 GPa) and high porosity (>20%). In addition, the SiCNH and SiCOH film also required an adhesion layer (SiO2) that has higher k and increase the overall effective dielectric constant. Furthermore, the reduction of SiCNH cap thickness is difficult due to SiCOH and porous SiCOH dielectric is a poor Cu diffusion and oxidation barrier.
In order to maintain or even reduce the interconnect capacitance at the decreasing dimensions of later technology nodes, the development of new high modulus composite copper cap and dielectrics with reduced dielectric constants are required to achieve lower effective k but with excellent mechanical modulus properties, enhance oxidation barrier and copper diffusion barrier properties. By adjusting the PECVD conditions and using the same precursor chemistry as for the copper cap deposition, a new SiCNxH/porous SiCNyH composite layer of copper cap/interlevel dielectric has been developed for high modulus and low effective k, where both copper cap and interlevel dielectric layer can act as good oxidation and copper diffusion.